Issue #755 - Make `sample_to_quantile()` require a `forecast` object

[nikosbosse]

Description

This PR closes #755.

This PR

• Forces the input to be an object of class forecast_sample and calls as_forecast() on the output as well
• makes the documentation for the internal sample_to_interval_long() depend on sample_to_quantile().
• updates the scoringutils.Rmd vignette to avoid an error.
• updates tests for both functions

Checklist

• [x] My PR is based on a package issue and I have explicitly linked it.
• [x] I have included the target issue or issues in the PR title as follows: issue-number: PR title
• [x] I have tested my changes locally.
• [x] I have added or updated unit tests where necessary.
• [x] I have updated the documentation if required.
• [x] I have built the package locally and run rebuilt docs using roxygen2.
• [x] My code follows the established coding standards and I have run lintr::lint_package() to check for style issues introduced by my changes.
• [ ] I have added a news item linked to this PR.
• [ ] I have reviewed CI checks for this PR and addressed them as far as I am able.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.30%. Comparing base (6279b62) to head (aaf6815).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #770 +/- ## ======================================= Coverage 95.29% 95.30% ======================================= Files 21 21 Lines 1595 1598 +3 ======================================= + Hits 1520 1523 +3 Misses 75 75 ```

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